Such a tire, generally intended to bear heavy loads, comprises a radial carcass reinforcement and a crown reinforcement which is composed of at least two working crown plies formed by inextensible reinforcing elements crossed from one ply to the next and forming equal or unequal angles, between 10 and 45°, with the circumferential direction.
The crown reinforcements of radial tires, more particularly with regard to tires having very large dimensions, are subjected to large deformations which give rise to longitudinal and transverse shear stresses between the edges of two plies (the longitudinal shear is greater than the transverse shear when the cords of crossed plies form small angles with the circumferential direction) at the same time as a delaminating stress, a radial stress tending to radially separate the edges of the two plies. The said stresses are due firstly to the tire inflation pressure, which has the effect that the so-called belt pressure between the carcass reinforcement and crown reinforcement tends to cause the circumferential expansion of the said crown reinforcement; the said stresses are also due to the load borne by the running tire when a contact area is established between the ground and tire; the said stresses are additionally due to the drifting of the running tire. These stresses produce cracks in the rubber compound adjoining the end of the shortest ply, these cracks propagating through the said compound and being detrimental to the durability of a crown reinforcement, and hence of the tire.
A clear improvement in the durability has been provided by using in the crown reinforcement at least one protective crown ply having an axial width greater than the width of the axially widest working ply.
In the case of a tire for “heavy goods” vehicles, a single protective ply is generally present and its protective elements are, in most cases, oriented in the same direction and with the same angle in absolute value as those of the reinforcing elements of the radially outermost, and hence radially adjacent, working ply. In the case of civil engineering tires intended for running over more or less uneven ground, the presence of two protective plies is advantageous, the reinforcing elements being crossed from one ply to the next.
Cords are termed inextensible when the said cords have, under a tensile force equal to 10% of the breaking force, a relative elongation at most equal to 0.2%.
Cords are termed elastic when the said cords have, under a tensile force equal to the breaking load, a relative elongation at least equal to 4%.
The circumferential direction of the tire, or longitudinal direction, is the direction corresponding to the periphery of the tire and defined by the running direction of the tire.
The transverse or axial direction of the tire is parallel to the axis of rotation of the tire.
The radial direction is a direction intersecting the axis of rotation of the tire and perpendicular thereto.
The axis of rotation of the tire is the axis around which it rotates during normal use.
A radial or meridian plane is a plane which contains the axis of rotation of the tire.
The circumferential mid-plane, or equatorial plane, is a plane perpendicular to the axis of rotation of the tire that divides the tire into two halves.
Under certain running conditions, the stresses prevail at the crown reinforcement, more particularly the shear stresses between the crown plies, and, combined with a significant rise in the operating temperature at the ends of the axially narrowest crown plies, result in the appearance and propagation of cracks in the rubber at the said ends, this in spite of the presence of a thickened rubber layer at the junction of the edges of working crown plies.
In order to overcome the above disadvantages and improve the durability of the crown reinforcement of the studied tire type, a number of prior patents claim solutions relating to the structure and quality of the layers and/or profiled elements of rubber compounds which are arranged between and/or around the ends of plies and more particularly the ends of the axially shortest ply.
Patent FR 1 389 428, with the aim of improving the resistance to degradation of the rubber compounds situated in the vicinity of the crown reinforcement edges, recommends the use, in combination with a tread of low hysteresis, of a profiled rubber element covering at least the sides and the marginal edges of the crown reinforcement and composed of a rubber compound with low hysteresis.
Patent FR 2 222 232, with the aim of avoiding separations between crown reinforcement plies, teaches coating the ends of the reinforcement in a pad of rubber, the Shore A hardness of which differs from that of the tread surmounting the said reinforcement and is greater than the Shore A hardness of the profiled rubber compound element arranged between the edges of crown reinforcement plies and the carcass reinforcement.
U.S. Pat. No. 5,154,217 uses a different unit of measurement and claims arranging between the ends of two plies, in the continuation of the ply contained between the said two plies, a cushion of rubber compound whose modulus of elasticity at 100% elongation is greater than the same modulus of the tread.
To join the two respective edges of two crown reinforcement plies, Patent FR 2 298 448 describes the use of so-called shearing rubbers between the said edges having a high Shore A hardness and high modulus of elasticity at 100%, this use being combined with the use of strips of anisotropic rubber arranged between the edges of the crown reinforcement and the carcass reinforcement.
The same applies in the case of joining the crown reinforcement plies described in Patent FR 2 499 912, the lateral part of the rubber layer arranged between the two main plies of the crown reinforcement being composed of a rubber compound having a high Shore hardness.
In Patent EP 1 062 106, the Applicant proposed a tire whose crown reinforcement comprises at least two plies of reinforcing elements, in which tire the edge of the axially narrowest ply is separated from the axially widest ply by a profiled rubber to compound element, the axially outer end of which is situated at a distance from the equatorial plane of the tire which is at least equal to the distance separating the said plane from the end of the widest ply, and the said profiled element being itself separated from the narrowest liner of the ply by an edging rubber, the said profiled element, the said edging rubber and the said liner having respective tensile secant moduli of elasticity at 10% relative elongation such that they decrease radially inwardly from the liner to the profiled element.
It has been found that the various structures listed above did not provide a completely satisfactory solution under highly punishing running conditions for the tire.
Another solution such as described in Patent FR 2 421 742 involves more favorably distributing the stresses which give rise to separation between working crown plies, following the drifting of the tire, by multiplying the number of working plies.
Multiplying the working plies is not without drawbacks, particularly at the centre of the crown reinforcement where the number of plies has a direct influence on the flexural rigidity of the crown of the tire. When this rigidity increases, it follows that impacts occurring on the crown of the tire, such as, in particular, when passing over large-sized stones, can lead to irreparable damage to the tire on account of this increased rigidity.
Patent Application WO 00/54992 has additionally proposed, with the aim of avoiding these drawbacks, producing a working crown reinforcement composed of at least three continuous working plies and of at least one half-ply, on each side of the circumferential mid-plane, which is arranged between the edges of at least two radially adjacent continuous working plies and whose distinguishing feature is particularly that it presents an angle with the circumferential direction that is greater than 25° and greater by a quantity of between 5° and 15° than the largest of the angles of the continuous working plies. The results obtained with this type of architecture were wholly satisfactory for the tested tire sizes.
The tests carried out with tires of this type show that, in light of the current requirements with regard to the considered applications, it may still be necessary to improve the performance of the tires particularly in terms of durability.